1. Field of the Invention
The present invention relates to a stepping motor driving apparatus, and more particularly, to a technology of driving a stepping motor with low noise and low vibration.
2. Description of the Related Art
Up to now, a sheet transporting device within a copying machine uses a stepping motor that enables open-loop speed control and position control with high precision. A sheet transporting mechanism performs control so as to drive transport rollers located at a sheet transport path by multiple stepping motors at predetermined timing so that a sheet reaches a predetermined position at predetermined timing and predetermined speed. The multiple stepping motors repetitively start and stop within the copying machine at the same time. Because a loud noise of each motor leads to a big noise of the entire machine, it is desired to reduce the noise and vibration of the motors.
FIG. 8A is a configuration diagram of a stepping motor driving apparatus that drives a two-phase stepping motor 310 (Japanese Patent Application Laid-Open No. H09-219995). The stepping motor driving apparatus includes first to eighth switching elements 101 to 108, a phase-A coil 300 and a phase-B coil 301 for the stepping motor 310.
A two-phase excitation that is a method of driving the stepping motor is exemplified. According to input phase signals, constant-current control circuits 500 and 501 control the switching elements (FETs) 101 to 108 so that rectangular wave pulse currents different in phase by 90 degrees from each other pass through the coil 300 of the phase A and the coil 301 of the phase B, respectively. As illustrated in FIG. 8A, a coil current is allowed to flow by one H bridge circuit per one phase of the coils of the stepping motor. Hereinafter, the drive current control will be described with reference to the H bridge circuit of the phase A. FIG. 8B illustrates an example of a pulse current waveform that flows through the coil 300. A flowing direction of a coil current Ic1p switches over every one-pulse period Tp, and constant-current chopping described later is conducted every chopping cycle Tchop1p (FIGS. 9A and 9B) which is sufficiently shorter than the pulse period Tp so that the coil current Ic1p coincides with a set current value It1p. For example, the chopping cycle Tchop1p is a cycle of one-twentieth ( 1/20) of the one-pulse period Tp. Similarly, when the coil current Ic1p is negative, control is made to bring an absolute value of the coil current Ic1p close to the set current value It1p. 
The constant-current chopping will be hereinafter described with reference to FIG. 9A. FIG. 9A is an enlarged diagram illustrating the coil current Ic1p of FIG. 8B. In the constant-current chopping, the constant-current control circuit 500 switches over the switch states of the switching elements 101 to 104 in order to switch over a current path that flows through the coil 300, in the chopping cycle Tchop1p. The constant-current control circuit 500 feeds to the coil since the start of chopping cycle Tchop1p until the coil current Ic1p reaches the set current value It1p. When the coil current Ic1p reaches the set current value It1p, the constant-current control circuit 500 enters a decay period in which the coil current Ic1p is decayed so as not to exceed a target current. The decay period is terminated after a time of the chopping cycle Tchop1p has passed since the feed start. The feed again starts, and a new chopping cycle Tchop1p starts.
A path along which an electric current flows through the coil 300 will be described. In this example, for simplification, a description is limited to a case where the electric current that flows through the coil is in one direction. However, when the direction of the electric current is opposite, the configuration is substantially identical with that in the former except that the on/off relationship of the switching elements 101 to 104 is axisymmetric. The constant-current control includes a feed period, a low decay rate period, and a high decay rate period. The current paths correspond to the respective periods, and the switching elements 101 to 104 are controlled so as to produce those current paths. Each chopping cycle Tchop1p is controlled to provide the feed period and the decay period, to thereby conduct the constant-current control. As a result of feed to the coil during the feed period, when the coil current Ic1p exceeds the set current value It1p, the feed period switches to the decay period. The feed period continues until the coil current Ic1p exceeds the set current value It1p. For that reason, as illustrated in FIG. 9B, a ratio of the feed period to the chopping cycle Tchop1p changes depending on when the coil current Ic1p reaches the set current value It1p. In the feed period within the chopping cycle Tchop1p, the switching elements 101 and 104 are turn turned on and the switching elements 102 and 103 are turn turned off, to allow an electric current to pass through a path connecting the switching element 101, the coil 300, and the switching element 104. Because the coil current Ic1p during the feed period flows through a current detection resistor 410, a voltage across the current detection resistor 410 is detected to detect the coil current Ic1p. 
A low decay rate path will be hereinafter described. The low decay rate path is a path through which an electric current flows only within the H bridge circuit, and connects the coil 300, the switching element 103, the switching element 104, and the coil 300. The low decay rate path is formed by turning on only the switching element 103. When the path switches over from the feed path to the low decay rate path, a back electromotive force occurs in the coil 300, and an electric current is allowed to pass through the coil 300 in a direction opposite to that of Ic1p during the feed period. Although the switching element 104 is turned off, an inverse parallel diode is disposed between the source and the drain of the switching element (FET), and hence an electric current flows through the inverse parallel diode of the switching element 104, with the result that the electric current flows through the low decay rate path. In this way, in the low decay rate path, only the resistive components of the coil 300 and the switching element 103 and the voltage drop component caused by the inverse parallel diode included in the switching element 104 are responsible for decaying the electric current, and hence the electric current gently decays.
A high decay rate path will be hereinafter described. The high decay rate path is a path along which the coil current Ic1p flows so as to charge a power source 100, and connects the switching element 104, the coil 300, and the switching element 101. The high decay rate path is formed by turning off all of the switching elements 101 to 104. When the path switches over from the feed path to the high decay rate path, a back electromotive force occurs in the coil 300, and an electric current is allowed to pass through the coil 300 in a direction opposite to that of the coil current Ic1p during the feed period. Although all of the switching elements 101 to 104 are turned off, an inverse parallel diode is disposed between the source and the drain of the switching element (FET), and hence an electric current flows through the inverse parallel diodes of the switching elements 101 and 104, with the result that the electric current flows through the high decay rate path. In this way, the high decay rate path enables an electric power to be regenerated in the power source 100, and allows a power supply voltage to be applied so as to reduce the electromotive force of the coil 300. Therefore, the coil current Ic1p of the coil 300 decays at a higher rate than that of the decay caused by the low decay rate path.
The current decay is achieved by a method in which any one of the low decay rate path and the high decay rate path is used, and another method in which the high decay rate path is used for decay when the decay period starts, and the high decay rate path is switched over to the low decay rate path at predetermined timing during the decay period.
The above-mentioned operation is conducted at the coil 301 similarly, and the respective electric currents that pass through the coils 300 and 301 are subjected to constant-current control according to phase signals that are input to the constant-current control circuits 500 and 501, to thereby conduct the constant-current control of the stepping motor 310.
Further, Japanese Patent Application Laid-Open No. 2007-104839 proposes an example of the stepping motor driving apparatus that aims at more silently operating the stepping motor. In this proposal, in the constant-current control, the decay path during the decay period is switched over to any one of the high decay rate path and the low decay rate path according to whether or not a coil current value Ic1pM reaches a target current value It1pM after a predetermined feed period, to perform the current decay.
However, when the current decay is conducted by using only the low decay rate path, the electric current may not be sufficiently decayed during the current decay period due to the back electromotive force developed in the coils according to the positional relationship of a rotor and a stator, resulting in a first problem that a copper loss of the motor becomes larger due to an increase in coil current caused by the back electromotive force. Further, when only the high decay rate path is used for the current decay in order to perform sure current decay, a current ripple becomes always larger, resulting in a second problem that an iron loss generated in the motor becomes larger. Further, when any one of the high decay rate path and the low decay rate path is used for the decay path during the decay period according to whether or not the coil current Ic1p reaches the target current value It1p to perform the current decay as described above, there is a case in which the current ripple becomes larger. This is because any one of the decay paths is used in one overall decay period. The motor torque is determined according to an angle of the rotor with respect to the excitation direction of the motor and the coil current. The current ripple generated in a very short cycle with respect to the rotation of the rotor changes the torque at high speed, causing a third problem that the torque fluctuation results in noise and vibration.